WASHINGTON -- President-elect Bill Clinton declared yesterday that "change is on the horizon." But exactly what that change will be isn't clear.

Indeed, as they peer into the first, faint light of the dawning administration, even some staunch Clinton supporters and advisers aren't sure just what the new president will do.

In his long campaign, Mr. Clinton may have put forth more ideas than any national candidate in history. The 232-page book he published last summer fails to contain them all.

Yet he was often purposefully vague on essential details. Mr. Clinton often said his agenda would have to fit the actual conditions in the country next Jan. 20, when he takes office.

But veterans of past administrations advise that he cannot afford to wait until his inauguration to begin writing the history of his presidency.

The 76-day transition phase, which began yesterday, "is the critical period in an administration," says Stuart Eizenstat, domestic policy chief in the last Democratic White House and a potential Clinton appointee. "When things are blundered or blown in a transition, you can take a large step toward defeating yourself before you are inaugurated."

The president-elect seems to have taken that advice to heart. His election-night victory speech sounded suspiciously like an inaugural address. It contained a sweeping call for "a new beginning" (the theme of President Ronald Reagan's 1981 inaugural). It also touched on a host of issues, including AIDS, defense conversion, the environment, campaign finance and welfare reform, student loans and national service.

But Clinton aides, aware that any new president risks failure if he attempts to do too much too fast, insist he'll focus on only a few priorities during his first 100 Days. They are:

* Reforming health care. perhaps his best chance to dramatically affect the lives of Americans. This is also the most complex domestic issue he faces and the supreme test of his ability to achieve meaningful change. Knowing that, Mr. Clinton had hoped to wait until after the inauguration to come up with a plan. But he did offer a fuzzy proposal during the campaign. It would limit total health spending while spreading health insurance benefits to all by the year 2000.

* Education. The focal point of his 12 years as governor of Arkansas. Mr. Clinton has proposed a national service plan that would allow students to repay college loans by performing community service, for example, by taking jobs as teachers or policemen for two years. He also proposes to make Head Start available to all poor preschoolers and wants to require private employers to set aside 1.5 percent of their payroll for job retraining.

* Economic recovery. Easily the top priority and the one that will determine whether there is money available to finance his other plans -- and perhaps the ultimate success of his presidency. His agenda in this area includes higher taxes on the wealthiest Americans and $20 billion in additional spending on roads, bridges and highways, and possibly high-speed rail and environmental technology to help stimulate the economy. He is also expected to propose a deficit reduction blueprint that would be used once the economy picks up.

Upon taking office, Mr. Clinton is also expected to make major changes, using his executive authority to rescind controversial social policies from the Reagan and Bush administrations. These include wiping out the so-called "gag rule" that prohibits anyone but doctors at government-funded clinics to mention abortion, ending the policy that bars U.S. aid to overseas organizations that promote abortion, removing the ban on the use of fetal tissue from abortions in medical research at the National Institutes of Health and perhaps dropping the import ban on the so-called abortion pill, RU-486.

As president, he will immediately set about to reverse the conservative tilt of the federal bench. There are signs that as many as three vacancies on the Supreme Court may occur during his term, in addition to an estimated 100-plus slots on the federal bench that he will be able to fill starting in January.

Also, a number of bills that were passed by Congress and vetoed by President Bush are expected to receive early action. Approval of a family leave law and a campaign finance reform measure are among those that Democratic leaders are expected to push immediately, to prove to the public that the gridlock in Washington has ended.

"There will probably be high expectations," said Sen. Joseph I. Lieberman, D-Conn., who has known Mr. Clinton since his days at Yale Law School in the early 1970s. "He can't change the world in three months."

Mr. Lieberman, the first senator outside Arkansas to endorse the Clinton candidacy, says that if the new president makes progress on most of the above agenda, "that would be a helluva start."